
MC68HC908EY16A MC68HC908EY8A Data Sheet, Rev. 2
Freescale Semiconductor
89
Figure 8-3. Internal Clock Generator Block Diagram
8.3.2.1 Digitally Controlled Oscillator
The digitally controlled oscillator (DCO) is an inaccurate oscillator which generates the internal clock
(ICLK). The clock period of ICLK is dependent on the digital loop filter outputs (DSTG[7:0] and DDIV[3:0]).
Because of only a limited number of bits in DDIV and DSTG, the precision of the output (ICLK) is restricted
to a precision of approximately
0.202 percent to 0.368 percent when measured over several cycles (of
the desired frequency). Additionally, since the propagation delays of the devices used in the DCO ring
oscillator are a measurable fraction of the bus clock period, reaching the long-term precision may require
alternately running faster and slower than desired, making the worst case cycle-to-cycle frequency
variation
6.45 percent to 11.8 percent (of the desired frequency). The valid values of DDIV:DSTG range
from $000 to $9FF. For more information on the quantization error in the DCO, see
8.4.4 Quantization8.3.2.2 Modulo N Divider
The modulo N divider creates the low-frequency base clock (IBASE) by dividing the internal clock (ICLK)
by the ICG multiplier factor (N), contained in the ICG multiplier register (ICGMR). When N is programmed
to a $01 or $00, the divider is disabled and ICLK is passed through to IBASE undivided. When the internal
clock generator is stable, the frequency of IBASE will be equal to the nominal frequency (fNOM) of 307.2
kHz
25 percent.
DIGITALLY
ICLK
TRIM[7:0]
VOLTAGE AND
CURRENT
REFERENCES
DIGITAL
++
+
–
– –
N[6:0]
DSTG[7:0]
FICGS
ICGEN
IBASE
DDIV[3:0]
LOOP
FILTER
CONTROLLED
OSCILLATOR
FREQUENCY
COMPARATOR
CLOCK GENERATOR
MODULO
N
DIVIDER
NAME
CONFIGURATION REGISTER BIT
REGISTER BIT
MODULE SIGNAL
NAME
TOP LEVEL SIGNAL